A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism. The tap changer is a device comprising a tap selector and a diverter switch which is able to move fast between different contacts, where transition resistors are used to mitigate the transition state when the switch moves between different contacts when the tap changer is on-load (without halting the operation of the transformer). The tap-changer can also be designed with the diverter switch and tap selector combined (selector switch type).
There is a need to detect whether the tap changer has successfully switched to the new contact, or if the tap changer has e.g., gotten stuck during the transition between contacts. If a fault due to unsuccessful switching is detected, the transformer may be tripped and thus disconnected. However, to trip the transformer is costly and it may take some time to get it online again. It is thus important to not trip the transformer unless necessary.